Light emitting devices and diodes are based on a forward biased p-n junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative incandescent and fluorescent lighting technologies. The efficiencies of LEDs can be quantified by three main factors, internal quantum efficiency, injection efficiency, and the extraction efficiency. The latter being the basis for the present invention. Several other factors affect the overall efficiency of solid state lighting applications such as phosphor conversion efficiency and electrical driver efficiency. However, these are beyond the scope of the present invention.
It is also of particular interest to maintain the small format light emitting device at a low temperature during operation as the junction temperate of the LED dramatically affects both its life time and its overall efficiency. As a basic rule, every 10° C. increase (above 25° C.) in junction temperature reduces the life time of the LED by 10 kHrs for a Galium Nitride LED. It is also a consequence of the increase of the junction temperature that the overall efficiency of a state of the art vertical type LED drops, for example, increasing the junction temperature from 40° C. to a 70° C. will reduce the efficacy of the LEDs by more than 10%. It is noted that this effect increasingly becomes nonlinear in behaviour. Thus, appropriate packaging solutions need to be developed to ensure performance is maintained and the operating temperature of the light emitting device is maintained for a given change in the junction temperature as well as the ambient temperature.
The thermal resistance of a package is the measure of how well a package can conduct heat away from the junction of the LED. Current state of the art modules have a thermal resistance of between 4 and 8 K/W.
Many methods have been successfully employed to improve the thermal resistance of LED module packages. These include the use of shaped metal lead frames in array formats (in U.S. Pat. No. 6,770,498), the use of bulk Aluminium Nitride ceramic tiles with electrical tracking on top (in published U.S. Patent Application 2006/0091415A1) and the use of flip chip LEDs onto tracked ceramic tiles with through vias to allow surface mounting (in published U.S. Patent Application 2006/0091409A1).
The LEDs themselves have been engineered to produce a low thermal resistance path from the junction to the package where the heat is spread, such as the flip chip approach described above (published U.S. Patent Application 2006/0091409A1) where the junction is very close to the package.
Another approach to provide LEDs with high current and thermal driving capabilities the vertical type n-p contact configuration in GaN material systems has been recently adopted an example of which has been disclosed in U.S. Pat. No. 6,884,646 and published U.S. Patent Application 2006/0154389A1. These use high thermal conductivity materials such as Copper to provide low thermal resistance from the junction to the package. More recently, improvements to these vertical type LED designs with respect to optical extraction performance promise even greater wall plug efficiency chips as described in UK patent applications 0704120.5 and 0714139.3.
U.S. Pat. No. 7,196,354 describes the introduction of a thermally conductive region in contact with the wavelength converting region and which comprises a material having a thermal conductivity greater than that of the wavelength converting element. In this case the thermally conductive material is optically non-transmissive designed to reflect the wavelength converted light. This leads to cumbersome additional reflective surfaces being introduced to re-direct and emit the wavelength converted light. Additionally, a larger light emitting package is required to accommodate the additional reflective thermally conductive surfaces. It is also not desirable to introduce reflective surfaces in the path of the emitted light as this may introduce optical loss affecting the overall efficiency of the LED. Additionally, any optical loss will ultimately lead to increased phonon vibrations leading to increased thermal load in the device.
Recently, the metal core printed circuit board (MCPCB) has been successfully employed for the implementation of LED Chip-on-Board (COB) lighting modules for improved thermal dissipation as well as reduce manufacturing cost. Different forms of LED COB modules have been proposed, for example in published U.S. Patent Application 2008/0084699, U.S. Pat. No. 7,176,502 and published International patent application WO 2007/086668 A1. These include secondary optics to provide modified far field emission from the LED module.
Additionally, high-thermal Conductivity MCPCB has also been proposed in UK patent applications 0716386.8. These employ electrical isolation layer having high thermal conductivity properties improving the vertical thermal dissipation properties of the circuit board.
Notwithstanding the developments in this field, there is still a need for a low cost LED Chip-on-Board type module that is optimised for improved thermal properties and enhanced brightness.